President Donald Trump and first lady Melania Trump escort former President Barack Obama and Michelle Obama as they leave the U.S. Capitol after the inauguration ceremony in Washington, Jan. 20, 2017. (STEPHEN CROWLEY / NYT)

Barack Obama left the presidency with job approval ratings in the upper 50s and a lengthy list of achievements ranging from the Affordable Care Act to new relationships abroad with traditional pariahs like Cuba and Iran.

Trump has fulfilled those expectations with a vengeance, perhaps propelled by the animus stirred when Obama mocked his birther crusade. Trump went further than new presidents traditionally do in seeking to overturn his predecessor's feats. After all, Republicans railed for 15 years against Social Security, but the next GOP president, Dwight Eisenhower, actually expanded it. Similarly, President Ronald Reagan acquiesced in most Great Society measures, though he cut some funding.

Many reversals stirred sharp international reactions, like the riots that greeted Monday's formal move of the U.S. embassy in Israel to Jerusalem and the condemnations from longstanding U.S. allies of his earlier withdrawals from global initiatives on climate change, trans-Pacific trade and the Iran nuclear program. Others stirred intense resistance at home, including his efforts to repeal the Affordable Care Act, weaken financial reforms, deport illegal immigrants given political sanctuary or brought here as children, and reverse the diversification of the federal judiciary.

In the end, the ultimate historical verdict on both presidencies may depend on whether Trump follows up his initial actions with acceptable replacements — a missing ingredient in many areas so far — and whether a future Democratic administration might restore the prior policies.

Trump often never consulted government experts nor U.S. allies. Last week's decision to fulfill his campaign pledge to withdraw from the multi-national agreement curbing Iran's nuclear program exemplifies his unilateral approach. And though he has talked broadly of replacing the existing pact with another more extensive one, Iran has shown no interest in that.

At home, the problems posed by Trump's failure to propose acceptable alternatives were underscored by the collapse in the Senate of the repeated GOP vow to "repeal and replace" Obamacare. He has implemented an array of administrative measures that surveys show have cost 3.2 million Americans their health coverage.

That pattern has persisted elsewhere. When Trump announced U.S. withdrawal from the Trans-Pacific trade partnership, he talked of negotiating replacement pacts with the 11 other participants. But they decided instead to proceed with a revised version of the original pact, which they signed in March.

Similarly, when Trump, who has called climate change a "con job" and a "myth," announced plans to withdraw from the multi-national Paris accord that Obama helped to negotiate, he talked vaguely of some replacement agreement. Again, the other participants ignored him.

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As a candidate, Trump denounced Obama for using executive authority in many areas, a step the former president justified because of congressional intransigence. But Trump has done the same thing, for the same reason.

One prime area is immigration, starting with his controversial order to ban immigration from eight predominantly Muslim countries and more recently orders ending the protection of immigrants from El Salvador, Haiti, Honduras, Nicaragua and Sudan, many fleeing political persecution.

His highest-profile move was last September's order ending the Deferred Action for Childhood Arrivals program Obama created to protect some 800,000 undocumented people brought to the U.S. by their parents when they were children.

Federal courts have so far blocked Trump's order, and legislative efforts to resolve the issue collapsed. Ultimately, the Supreme Court will decide.

Trump has actually benefited from Obama's most enduring achievement: the much-criticized economic stimulus plan that helped end a deep recession and began the lengthy expansion that continues to buoy the economy today. One goal of the tax cut that was Trump's biggest 2017 legislative accomplishment was to extend that expansion.

Other Obama accomplishments remain, such as government recognition of same-sex marriage, which was subsequently legalized by the Supreme Court, allowing gays and lesbians to serve openly in the military, revival of the U.S. auto industry and diversification of the federal judiciary. On Obama's watch, al-Qaeda leader Osama bin Laden was killed and U.S. combat operations ended in Iraq and Afghanistan.

Hillary Clinton's 2016 defeat opened the way for Trump to reverse many other Obama actions. But some of Trump's reversals could themselves be at risk if Democratic victories in the 2018 congressional election and the 2020 presidential contest give them the power to trump them.

Carl P. Leubsdorf is the former Washington bureau chief of The Dallas Morning News and a frequent contributor. Email:carl.p.leubsdorf@gmail.com